Uninterrupted power supply (UPS) systems are widely used in contexts where having an electrically stable and reliable power supply is critical for the operation of equipment and systems. UPS systems are typically of the standby, line interactive, or online double conversion design. These three UPS designs provide different levels of protection from electrical power disturbances, such as voltage surges, or failures, such as black outs. As an example, many data centers use online double conversion UPS designs that provide a high level of protection.
An online double conversion UPS system typically draws alternating current (AC) power from a commercial power grid and converts the AC to direct current (DC). The DC supply is used to keep a battery fully charged and is converted back to AC in an inverter to deliver clean, regulated AC power as required by a customer's equipment. The battery is of sufficient capacity to provide backup power for the equipment being protected. If there is an interruption to the supply of AC power to the UPS, energy stored in the batteries is used to feed the inverter which will continue to provide the required AC power for the customer's equipment for a predetermined period of time.
To ensure a high level of reliability, UPS manufacturers conduct extensive testing at their factories and also at installation sites during initial setup and commissioning. UPS systems are also tested periodically by end users to ensure their continued reliable operation. These tests typically include full load testing using a resistive load bank. Power from the UPS is delivered to the resistive load bank over a lengthy test period, such as a period from one to four hours. The UPS output is converted to heat in the resistive load bank which is dissipated to the surrounding air. As a consequence, the resistive load bank requires adequate ventilation, such as may be provided by mechanical ventilation equipment, to dissipate the heat and must be kept away from combustible material and vapors. Since a large number of the electrical industry and user groups use resistive load banks for testing electrical equipment, such as UPS systems, a significant amount of energy is lost performing the tests. Consequently, manufacturers, as well as users of UPS systems, incur significant costs to pay for the energy used during testing, and may further incur a demand charge that is based on the time of the day when a test is run.